Chris Benz and Dave Shireley will be filing dispatches from the Rugby World Cup in New Zealand, in…
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The answer is the "choke," a tackling strategy created by Les Kiss, the Irish defensive coach, that exploits a loophole in rugby rules. The Wall Street Journal (among others), explains:

What makes the choke so effective is the way it has reinvented one of this game's oldest and most established arts: the tackle. For more than a century, rugby players have used the same basic technique to take down opponents: Hit them low and mash them into the turf as quickly as possible. Even as teams employ bigger, faster payers, the fact that they fall over if you hit them low enough has remained true.

For Ireland, the reverse is true. Instead of trying to take down opposing ball carriers, this team of muscular brutes wants to hold runners upright and prevent them from hitting the ground with a tandem of chest-high tackles-roughly akin to receiving a bear hug from a boa constrictor.

"You dare not leave runners isolated in the tackle," said Ben Kay, a former England international and now a rugby analyst for ESPN. "They're like a pack of dogs."

Executing the choke requires two defenders to tackle a ball carrier from either side. One goes high, trapping the runner's arms against his body, the other goes low, preventing him from reaching the ground.

The physical toll required to perform the choke is so exhausting that it would have been impossible to implement without the fitter, faster, physically stronger players in today's game or the time to practice it obsessively which is now available in the professional era. But when it works, it traps opposing runners in a sort of suspended animation.

When the Irish have their opponent choked, they try to strip the ball. They also win possession if a scrum results. The strategy appears to be a genuine innovation in the way rugby will be played and, likely, governed in the future. Other teams are already copying Ireland.

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Where one loophole closes, however, another opens. In 2009, the rugby rules changed to penalize defenders who held an opponent down in an attempt to steal the ball, a tactic also embraced by Ireland. The idea was to create the conditions for longer, more exciting runs. Instead, the new rules begat the choke, which prevents runners from hitting the ground altogether. This, of course, is how Sport evolves: a cheeky underdog facing long odds bends the rules and, eventually, redefines them.